Suit also important
I think we have to be clear on "Knights" vs. "Knight/Swords." Knights, in general, are as nisaba said, young men on quests with the best of intentions and the noblest actions, though even upright, their youth can make them focused so much on those intentions that they charge ahead without considering the consequences. Like Starry's mention of tilting at Windmills. Keep that in mind, too. Whatever their age, they are thinking and acting more like teenagers than adults. Which is why, when reversed, they all have chips on their shoulders
However, we also need to be clear of the suit, as it tells us more about our knight's individual personality, what they'll tilt at and how they'll go wrong when reversed.
Knight/Swords is in that "intellectual" suit, and so even upright he's going to be less about those emotional and passionate pursuits that we attribute to the Camelot sort of knight. Less interested than the Knight/Cup at winning a fair maid's favor or being selfless and kind to the weak, less interested than the Knight/Wands with being the grand hero, winning tournaments or performing dramatic gestures, less interested than the Knight/Pents at getting that treasure back to the villagers from the dragon or like Percival, doggedly pursuing his goal for months on end.
Knight/Swords is trying to show off his smarts. He wants the King to say, "Sir, Sword, you are the cleverest of all my Knights!" So if he's going to tilt at a windmill, it needs to be a difficult puzzle to solve. And that old lady he's helping across the street probably has some fascinating information he's interested in. It's not that Knights are selfish, just focused on their goal and on winning the prize, whatever that prize may be.
Turn a Knight upside down and you've got two possibilities. Either he's blocked from his goal--meaning our Knight/Swords is tongue-tied and can't think sharp and clear--or he's using his talents to undermine his kingdom rather than to bring glory to it. So what you have in a Knight/Swords rx is a verbally and mentally abusive boyfriend. His goal isn't to use his talent with words to make his partner feel special and wonderful--or use his cleverness to solve a problem in the relationship--but rather to undermine that partner with words and smarts. Likewise if this Knight/Swords is a co-worker or a friend, he will take all he knows of a person that he sees as competition and use it like a sword on them. He may spread rumors that sound so valid and true that they damage that person for a long time.
One place where you can see many a Knight/Sword rx is on internet forums, by the by. If you run into a Troll who is particularly clever, relentless and cruel, that's your Knight/Swords rx. They have enough maturity to make some nasty cuts (unlike a Page/Sword rx who can only toss out childish taunts and gossip), and they are as relentless as a Knight on a mission.